plugin_dir inconditionally defaults to "/usr" despite being set

Whereas plugin_dir being natively set to `/usr/lib/x86_64-linux-gnu/mariadb19/plugin` (confirmed by the output of the command show variables like 'plugin_dir';), adding any plugin in the section [mariadb] of the file _ /etc/mysql/mariadb.conf.d/50-server.cnf_ file and then running mysql_install_db results in the following error in the logs /var/log/mysql/error.log :

Can't open shared library '/usr/file_key_management.so (errno: 22, cannot open shared object file: No such file or directory)

The expected behavior would be to look into /usr/lib/x86_64-linux-gnu/mariadb19/plugin since this value is correctly (and by default) set.

To dig more, I tried to manually set the plugin_dir value in my.cnf and the result is the same.

The only (dirty) workaround I found at the moment is to copy all *.so from /usr/lib/x86_64-linux-gnu/mariadb19/plugin to /usr/